In many industries solid plates (also referred to as sheets) of polymer are made by pouring a liquid polymer onto a flat horizontal surface, letting the liquid polymer level under the influence of gravity and curing it to form a solid. The liquid "polymer" may be a polymerizable material which is not itself a polymer according to the literal meaning of the word polymer. Typically the material may be selectively solidified, or may have its solubility in a solvent altered, by exposure to actinic radiation.
Some polymers can be cured by exposure to actinic radiation such as ultraviolet (UV) light. When the curing is done by UV light, the polymer is referred to as a photopolymer. Photopolymers can be selectively cured by placing an exposed and developed film under the liquid polymer layer shining a light on the underside of the polymer layer and using an image on the film to control the amounts of light reaching different parts of the polymer. Flexographic printing plates and letter press polymeric printing plates can be prepared in this manner.
Prior art systems provide an ultraviolet light source which has dimensions approximately equal to those of the largest printing plate to be cured. The light source evenly illuminates an under side of a large support plate, which is typically a thick sheet of glass. To make a plate, liquid polymer is poured on an upper surface of the support plate. The upper surface of the support place provides a flat base surface for the polymer.
Such prior art systems have two main disadvantages. First, providing a large light source capable of evenly illuminating each portion of the support plate is expensive and is wasteful in cases where the polymer plates to be made are significantly smaller than the support plate.
Secondly, support plates must generally be thick to reduce sagging. Any large glass plate which is supported around its edges tends to sag in its center. The sag of the glass support plate causes the finished polymer plate to be thicker in its middle area. This is because the top surface of the polymer is always flat due to the liquid levelling itself, while the bottom follows the glass support plate. To achieve a desired thickness uniformity of, for example, 25 microns (0.001") over an area of a few square meters, special measures must be taken to support the weight of the glass. Thicknesses of 15 mm or more are common. A thick support plate tends to attenuate radiation which is passing through it. Therefore, a powerful illumination source is typically required for curing polymer printing plates.
An additional problem occurs when such prior systems are used to image photopolymer plates. Such systems typically offer little control over the cross sectional profiles of features in the imaged plates.
There is a need for apparatus and methods for curing photopolymer plates which are more efficient and less expensive than current methods and apparatus. There is also a need for a system which provides enhanced control over the cross sectional profiles of features in imaged photopolymer plates.